Method for Designing and Selecting Fiber for Use with a Transmitter Optical Subassembly

ABSTRACT

A method for compensating for both material or chromatic dispersion and modal dispersion effects in a multimode fiber transmission system is provided. The method includes, but is not limited to measuring a fiber-coupled spatial spectral distribution of the multimode fiber laser transmitter connected with a reference multimode fiber optical cable and determining the amount of chromatic dispersion and modal dispersion present in the reference multimode fiber optic cable. The method also includes, but is not limited to, designing an improved multimode fiber optic cable which compensates for at least a portion of the chromatic dispersion and modal dispersion present in the reference multimode fiber optic cable resulting from the transmitter&#39;s fiber-coupled spatial spectral distribution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/909,129, filed Oct. 21, 2010, the subject matter of which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a new Differential Mode Delay (DMD)specification which is provided for making and utilizing laser-optimizedmultimode fiber optic cable (MMF) that utilizes the radial dependence ofthe fiber-coupled wavelength distribution to compensate modal andchromatic dispersion for improved channel performance.

The present invention also relates to a multimode fiber opticsub-assembly having a specified fiber-coupled spatial spectraldistribution. The multimode fiber optic sub-assembly includes amultimode fiber transmitter optical sub-assembly (TOSA) for use with aspecifically designed MMF. Knowledge of the fiber-coupled spatialspectral distribution allows dispersive phenomena, inherent in multimodefiber optic communications system, to be compensated by specificallydesigned MMF thereby facilitating improved systems performance.

BACKGROUND

Most high-speed optical channel links in short-reach data communicationnetworks employ MMF. Transceivers that support these channel links useVertical Cavity Surface Emitting Laser (VCSEL) sources for data rates of1 Gb/s and higher. To achieve link distances in excess of 200 meters,the design of MMF has been optimized for VSCEL transmission with acenter wavelength of 850 nm. An MMF optimized for VCSEL transmission iscalled Laser-Optimized MMF, and is specified in TIA-492AAAC andTIA-492AAAD as OM3 (fibre type A1a.2) and OM4 (fibre type A1a.3) fibertypes respectively.

Due to the wave nature of light and the wave guiding properties ofoptical fiber, an optical signal traverses the fiber along discreteoptical paths called modes. The optical power of the pulse is carried bythe sum of the discrete modes. The difference in propagation delaysbetween the fastest and slowest modes in the fiber determines theinter-modal dispersion or simply modal dispersion. MMF should ideally beoptimized so that all modes arrive at the output of the fiber at thesame time to minimize modal dispersion. This has traditionally beenachieved by shaping or “grading” the refractive index profile of thefiber core according to the parabolic distribution defined by

$\begin{matrix}{{n^{2}(r)} = \left\{ \begin{matrix}{n_{1}^{2}\left\lbrack {1 - {2\Delta \; {n\left( {r\text{/}a} \right)}^{\alpha}}} \right\rbrack} & {r \leq a} \\n_{2} & {r > a}\end{matrix} \right.} & (1)\end{matrix}$

Where, a is the core diameter (50 μm), n₁ is the refractive index at thecore center, n₂ is the refractive index of the cladding, α is a numberclose to 2, and

$\begin{matrix}{{\Delta \; n} = {\frac{n_{1}^{2} - n_{2}^{2}}{2n_{1}^{2}}.}} & (2)\end{matrix}$

The traditional refractive index profile described by Equation (1)assumes that all modes have substantially the same wavelength and isconsidered the traditional “ideal” profile that results in minimum modaldispersion. Modes that travel with larger angles (and consequentlytraverse longer distances) encounter a lower refractive index on averageand travel faster. These are called high-order modes. Modes travelingwith small angles (low-order modes) encounter a higher refractive indexon average and travel slower.

Much attention has been focused on minimizing modal dispersion ofLaser-Optimized MMF by optimizing the refractive index profile of thefiber. Modal dispersion is the temporal distortion of an optical signaldue to differences in the various modes' propagation velocities.Conversely, with respect to MMF, comparatively little attention has beenfocused on reducing the effects of material dispersion. Materialdispersion is the temporal distortion of an optical signal due todifferences in the propagation velocities of the various spectralcomponents that comprise the optical signal. More generally, chromaticdispersion is a combination of material dispersion and waveguidedispersion where the waveguide properties change with wavelength.

As a result, it is desirable to provide an improved method formanufacturing MMF which accounts for and compensates for not only modaldispersion, but also material dispersion.

Additionally, due to the radially dependent wavelength emission patternof laser transmitters, fiber-coupled modes have a radial wavelengthdependency that results in appreciable material dispersion.Consequently, the total dispersion of the MMF system depends not only onthe modal dispersion and material dispersion within MMF but also on theinteraction between MMF and the emitting spectrum of the lasertransmitter (oftentimes a VCSEL), all of which is governed by afiber-coupled spatial spectral distribution.

The fiber-coupled spatial spectral distribution is dependent on theemitting spectrum of a laser transmitter which generates light radiationwhich travels down the MMF, and the manner in which the VCSEL'sgenerated light radiation is coupled into the MMF. With reference toFIGS. 15 and 16, a Transmitter Optical Sub-Assembly (TOSA) 120 is usedto couple light emitted from a VCSEL 124 into a fiber connector ferrule132 mated to a transceiver 112 which houses both the TOSA 120 and aReceiver Optical Sub-Assembly (ROSA) 130. The ROSA is used for lightdetection. With reference to FIGS. 15 and 16, most generally, a TOSAcomprises the following components: 1) a packaged VCSEL 124; 2) a lens126; 3) a precision receptacle 128 for receiving a removable fiberconnector ferrule 132; 4) a TOSA housing 121; and 5) an electricalconnection 123 to a transceiver PCB. The packaged VCSEL 124 is mostoften packaged in a hermetically sealed package to improve devicereliability. The lens 126 may be integrated into the packaged VCSEL 124or molded into the TOSA housing 121. An illustration of a transceivershowing the TOSA is provided in FIG. 15 and a cross-sectional schematicof a TOSA is provided in FIG. 16.

For illustrative purposes only, it may be considered that the componentsof the TOSA 120 must be carefully aligned to achieve acceptableperformance. An example assembly process for TOSA 120 may be summarizedby the following steps. First, secure the lens 126 to the TOSA housing121. Typically this is done by a press-fit, epoxy (thermal or UV cure)or laser welding. Second, position the TOSA housing 121 with lens 126over an electronically addressable VCSEL 124. Third, insert the fiberconnector ferrule 132 into the TOSA housing 121. Fourth, turn the VCSEL124 on. Fifth, align the VCSEL 124 with respect to the TOSA housing 121,including lens 126 and fiber connector ferrule 132, to achieve thedesired fiber-coupled power. Typically a 3-axis alignment is performed(x,y,z). Optical alignment of the TOSA 120 is typically achieved byinserting a fiber connector ferrule 132 into the receiving end of theTOSA 120 and optimizing for the maximum optical power as a function ofVCSEL 124-to-package placement. Sixth, secure the VCSEL 124 to the TOSAhousing 121. Typically this is done with epoxy (thermal or UV cure) orlaser welding. Finally, remove the fiber connector ferrule 132 from thecompleted TOSA 120.

The different transverse modes of multimode VCSELs have differentemission angles; higher-order modes have larger emission angles. It isalso known that higher-order VCSEL modes have shorter wavelengths. Whencoupled into multimode fiber, the spectrum of the higher-order fibermodes will have a reduced central wavelength, λ_(c) compared tolower-order fiber modes. The measurement procedure described inTIA-455-127-A may be used to measure the emitting spectrum and determineits central wavelength, λ_(c).

With reference to FIG. 17, when the components of TOSA 120 arepre-selected and aligned with tolerances within 1 mm or less,higher-order VCSEL modes are coupled into the higher-order fiber modeslocated farther from the center of the fiber core, λ_(c outer).Conversely, lower-order VCSEL modes, which also have a longer centralwavelength, are expected to be coupled into the lower-order fiber modeslocated near the fiber core center, λ_(c inner).

However, with reference to FIG. 18, if the components of TOSA 120 arenot precise and/or are in poor alignment, due to debris or misalignmentissues such as VCSEL offset within the TOSA package or lens 126 isoffset within the TOSA housing 121, the expected proportionalrelationship between VCSEL modes and fiber modes may not be realized. Infact, the optical system comprising the TOSA 120 may be such thathigher-order VCSEL modes are coupled into low-order fiber modes and viceversa. It is essential to recognize that although imprecise componentsand/or poor alignment may result in optical aberrations, thefiber-coupled power may still exceed the specification minimum.

Numerous conditions, including environmental ones, may result in such asituation. Some examples include: 1) Misplacement of the VCSEL withinthe TOSA package; 2) Debris in the optical path; 3) Lens defects (e.g.moderate radius of curvature, excessive radius of curvature); 4) Thermalexpansion (or contraction) of the various components comprising theTOSA; 5) Debris inside the ferrule bore preventing complete insertion;6) Excessive ferrule concentricity; 7) Excessive fiber concentricity;and 8) TOSA housing defects.

There exists a need to have a transceiver that produces a predeterminedfiber-coupled spatial spectral distribution that results in a materialor chromatic dispersion that can be readily compensated for with asingle fiber design. As a result, it would be desirable to provide animproved method for manufacturing a TOSA that produces a controlledfiber-coupled optical spectral distribution.

SUMMARY

In one aspect, a method for compensating for both material or chromaticdispersion and modal dispersion effects in a multimode fibertransmission system is provided. The method includes, but is not limitedto measuring a fiber-coupled spatial spectral distribution of themultimode fiber laser transmitter connected with a reference multimodefiber optical cable and determining the amount of chromatic dispersionand modal dispersion present in the reference multimode fiber opticcable. The method also includes, but is not limited to, designing animproved multimode fiber optic cable which compensates for at least aportion of the chromatic dispersion and modal dispersion present in thereference multimode fiber optic cable resulting from the transmitter'sfiber-coupled spatial spectral distribution.

In one aspect, a method for compensating for both chromatic dispersionand modal dispersion effects in a multimode fiber transmitter opticalsub-assembly is provided. The method includes, but is not limited tomeasuring a fiber-coupled spatial spectral distribution of the multimodefiber transmitter optical sub-assembly connected with a referencemultimode fiber optic cable and determining the amount of chromaticdispersion and modal dispersion present in the reference multimode fiberoptic cable.

In one aspect, a method for compensating for both chromatic dispersionand modal dispersion effects in a reference multimode fiber transmitteroptical sub-assembly is provided. The method includes, but is notlimited to, measuring a fiber-coupled spatial spectral distribution ofthe reference multimode fiber transmitter optical sub-assembly connectedwith a reference multimode fiber optic cable and determining the amountof chromatic dispersion and modal dispersion present in the referencemultimode fiber optic cable. The method also includes, but is notlimited to, designing an improved multimode fiber transmitter opticalsub-assembly, which compensates for at least a portion of the chromaticdispersion and modal dispersion present in the reference multimode fiberoptic cable.

In one aspect, a method for compensating for both chromatic dispersionand modal dispersion effects in a multimode fiber optic cable isprovided. The method includes, but is not limited to, generating anoptical signal into a reference multimode fiber optic cable andmeasuring a wavelength dependent time of flight for a plurality ofguided modes of the optical signal in the reference multimode fiberoptic cable. The method also includes, but is not limited to,determining the amount of chromatic dispersion and modal dispersionpresent in the reference multimode fiber cable, and designing animproved multimode fiber optic cable which compensates for at least aportion of the chromatic dispersion and modal dispersion present in thereference multimode fiber optic cable.

The scope of the present invention is defined solely by the appendedclaims and is not affected by the statements within this summary.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 depicts a graph of optical spectrums of modes propagating in aMMF for five radial offsets from the multimode fiber core center, inaccordance with one embodiment of the present invention.

FIG. 2 depicts a graph of propagation delay of a resultant output pulserecorded by a sampling oscilloscope for four radial offsets.

FIG. 3 depicts a graph of a DMD waveform plot for a representative OM4fiber sample A, in accordance with one embodiment of the presentinvention.

FIG. 4A depicts a graph of a current ideal DMD waveform plot where allwaveforms have the same delay and are therefore aligned, in accordancewith one embodiment of the present invention.

FIG. 4B depicts a graph of a resultant DMD waveform for a test VCSELplot obtained by convoluting the ideal DMD waveforms with spectral datausing a new algorithm, in accordance with one embodiment of the presentinvention.

FIG. 5 depicts a graph of a VCSEL emission pattern which has awavelength dependency that results in a radial wavelength dependency ofthe coupled fiber modes as a function of offset, as shown on rightmostchart, in accordance with one embodiment of the present invention.

FIG. 6 depicts a graph of wavelength dependent time of flight for tworadial offsets (5 and 19 μm offsets) propagating through a 550 m lengthof MMF.

FIG. 7 depicts a graph of relative delay derived from time of flightmeasurements.

FIG. 8A depicts a graph of a DMD waveform plot having a monotonicreduction in delay from 0 to 24 μm across the core of the fiber.

FIG. 8B depicts a graph of a resultant DMD waveform plot when convolutedwith a radial shift in center wavelength of Δλ_(c)=0.53 nm.

FIGS. 9A and 9B depict graphs of DMD waveform plots for two MMF's in thesame cable with virtually identical DMD and EMB metrics, in accordancewith one embodiment of the present invention. The fiber with a leftshift, plotted in FIG. 9A, shows better BER system performance than thefiber with the right shift, plotted in FIG. 9B.

FIGS. 10A and 10B depict graphs of calculated DMD waveform plots for thesame two fibers characterized in FIGS. 9A and 9B, respectively.

FIG. 11A depicts a graph of measured minEMBc using a standard testmethod, in accordance with one embodiment of the present invention.

FIG. 11B depicts a graph of a newly calculated and convoluted minEMBc,in accordance with one embodiment of the present invention.

FIG. 11C depicts a graph of measured DMD, Inner Mask specification,using a standard test method, in accordance with one embodiment of thepresent invention.

FIG. 11D depicts a graph of newly calculated and convoluted DMD, InnerMask specification, in accordance with one embodiment of the presentinvention.

FIG. 12 depicts a graph of calculated minEMBc for three transceiverswith different Δλ_(c), in accordance with one embodiment of the presentinvention.

FIG. 13 depicts a graph of DMD compensation required to maximize thecalculated fiber bandwidth for minEMBc, in accordance with oneembodiment of the present invention.

FIG. 14 depicts a graph of a fiber sample A 3 dB bandwidth measurementsusing five different transceivers, in accordance with one embodiment ofthe present invention.

FIG. 15 depicts a perspective view of a transceiver showing atransmitter optical sub-assembly (TOSA), in accordance with oneembodiment of the present invention.

FIG. 16 depicts a cross-sectional schematic of a TOSA, in accordancewith one embodiment of the present invention.

FIG. 17 depicts a cross-sectional schematic of a TOSA having componentswhich are precise and well-aligned, in accordance with one embodiment ofthe present invention.

FIG. 18 depicts a cross-sectional schematic of a TOSA having componentswhich are not precise and well-aligned, in accordance with oneembodiment of the present invention.

FIG. 19 depicts an experimental setup used to measure a fiber-coupledspatial spectral distribution for multimode fiber transceivers, inaccordance with one embodiment of the present invention.

FIG. 20 depicts a graph showing a measured fiber-coupled spectrum at twodifferent fiber core radii: r=0 μm and r=24 μm for a particulartransceiver, in accordance with one embodiment of the present invention.

FIG. 21 depicts a graph showing a change in calculated centralwavelength across a fiber core for a first and second transceiver, inaccordance with one embodiment of the present invention.

FIG. 22 depicts a histogram of the maximum difference in a fiber-coupledcentral wavelength across the core of a reference multimode fiber forthirty-eight short wavelength, high bit rate, multimode fibertransceivers, in accordance with one embodiment of the presentinvention.

FIG. 23 depicts a graph showing calculated probability distributions ofminimum optical link distances for: 1) links comprising TOSAs that donot specifically control fiber-coupled spatial spectral distribution anda fiber designed to generally compensate for this wide distribution; and2) links comprising TOSAs with specifically controlled fiber-coupledspatial spectral distributions and a fiber designed to preciselycompensate for material dispersion with modal dispersion, in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION

The present invention makes use of the discovery that due to theradially dependent wavelength emission pattern of VCSELs and the mannerin which light is coupled into the fiber, the fiber-coupled modes havespectral components that depend on fiber radii and result in a chromaticor material dispersion effect that cannot be neglected. FIG. 1illustrates the optical spectrums of the modes propagating in a MMF forfive radial offsets across the fiber core. The center wavelength, orcentral wavelength, for each radial spectrum is indicated by the downarrow. As shown in FIG. 1, for this particular optical transmitter, onaverage the central wavelength of a fiber mode shifts to a shorterwavelength for larger radial offsets. This radial wavelength dependencecauses fiber modes to undergo a chromatic or material dispersionrelative to each other in addition to modal dispersion. Consequently,the refractive index profile must be modified to compensate for thismaterial dispersion effect and thereby minimize the total dispersionrealized by a fiber and transmitter combination.

The present invention also makes use of the discovery that high-orderVCSEL modes may be coupled into high-order fiber modes as shown in FIG.17, by requiring more precise TOSA components and more precise alignmentof the TOSA components during the manufacturing process to take intoaccount the effects of material dispersion as well as modal dispersion.

In one aspect, the present invention describes a new DMD specificationfor laser optimized multimode fiber that compensates modal and materialdispersion effects caused by the radially dependent shift in wavelengthsof the fiber modes. The required compensation depends on the spatialspectral characteristics of the VCSEL, the coupling of the VCSEL modesinto fiber modes, and the modal dispersion properties of the fiber. MMFdesigned to compensate for the radial shift in wavelength will exhibitreduced total dispersion and improved system performance. Since currentDMD and Effective Modal Bandwidth (EMB) test methods neglect the radialwavelength dependence of the fiber modes, they do not accuratelycharacterize the modal dispersion or bandwidth performance realized in atransmission system. To verify the new DMD specification, we applied animproved algorithm for calculating DMD and bandwidth, as disclosed inU.S. Patent Application Ser. No. 61/237,827, to calculate the DMD andEMB for a sample set of VCSELs for varying amounts of materialdispersion compensation.

The test method used to characterize the bandwidth of laser optimizedMMF is DMD, specified in the TIA-455-220-A standard. DMD is a measure ofthe difference in propagation delay between the fastest and slowestmodes traversing a MMF expressed in units of ps/m, while the effect ofchromatic dispersion is minimized. The larger the relative delay betweenmodes, the larger the dispersion (i.e. modal dispersion). To measure theDMD, an optical reference pulse emitted from the end of a single-modelaunch fiber is stepped across the core of the MMF under test. For eachradial offset, the propagation delay of the resultant output pulse isrecorded by a sampling oscilloscope, as illustrated in FIG. 2.

The output waveform contains only those modes excited by the launchpulse for a given radial offset. In this measurement the spectralcharacteristics of the launch pulse remain constant. To compute the DMD,the set of output waveforms for each of the radial offsets (0 to 24 μm)are first recorded. A plot of the waveforms is shown in FIG. 3. The plotshows the relative pulse delay, or “relative time” in picoseconds permeter (ps/m) at the output end of the fiber (x-axis) as a function ofthe radial offset of a launch pulse (y-axis) as measured from the corecenter. The DMD is determined by measuring the difference in pulse delaybetween the leading edge of the fastest pulse and the trailing edge ofthe slowest pulse. From this difference we subtract the temporal widthof the launch pulse, which yields the modal dispersion of the fiber. Tospecify the fiber as OM3 or OM4, the DMD must meet minimum dispersionvalues within several radial regions of the core.

Currently, it is generally believed that the DMD waveform plot thatminimizes modal dispersion is when all the radially offset waveformsarrive at the output of the fiber at the same time as shown in FIG. 4A.This is true if each of the radial fiber modes (output waveforms) isexcited by launch pulses having the same spectral characteristics.However, since higher-order VCSEL modes emitted into larger launchangles have shorter wavelengths, on average, than those modes emittedinto smaller launch angles (see FIG. 5), that assumption is not valid.When VCSEL modes are coupled into the fiber, low-order modes andhigh-order modes undergo different amounts of material dispersion inaddition to modal dispersion. Material dispersion occurs because therefractive index of a material changes with wavelength, i.e.,

$\begin{matrix}{\frac{n}{\lambda} \neq 0} & (3)\end{matrix}$

Using the algorithm disclosed in U.S. Patent Application Ser. No.61/237,827, we calculated the DMD for the transmitter used in a BitError Rate (BER) test system for a simulated fiber having an ideal DMDwaveform. The difference in center wavelength across the core of thefiber for this VCSEL is 0.72 nm. Applying the new algorithm, we see thatthe calculated DMD waveforms (FIG. 4B) at larger radial offsets areshifted to the right (i.e. longer delays) due to material dispersion.Hence, the traditional “ideal” parabolic refractive index profile thatyields a DMD with all waveforms temporally aligned is not optimum forthis VCSEL. To minimize total dispersion (or maximize system bandwidth)the core refractive index profile of MMF must be modified to compensatefor the relative material dispersion (or chromatic dispersion) of thefiber modes. Given the spectral characteristics of a laser, the requiredcompensation can be calculated using the algorithm disclosed in U.S.Patent Application Ser. No. 61/237,827. The algorithm disclosed in U.S.Patent Application Ser. No. 61/237,827 provides a more accuratecharacterization of the system performance of a MMF channel link.

In general, VCSEL transceivers used in high-speed data communicationnetworks exhibit a spatially dependent wavelength emission pattern that,when coupled into fiber, produces a wavelength dependency in thesupported guided modes. Although other parameters to quantify the radialdependence of the fiber-coupled spectrum may be used, we define a newparameter Δλ_(c), to be the maximum difference in center wavelengthbetween the radial spectrums across the core of a MMF. Based on astatistical distribution of Δλ_(c) for a representative set of opticaltransceivers for 10 Gbps Ethernet (10GBASE-SR), and 8 Gb/s FibreChannel, a new DMD specification is proposed for the design andfabrication of laser-optimized MMF. The new DMD specificationcompensates for the radially dependent shift in wavelengths of theoptical channel. As an example, in FIG. 5 we show Δλ_(c) for a sample 10Gb/s transceiver, where, Δλ_(c)=0.53 nm. We note that if the coupledoptical power is restricted to a small region of the fiber core, thenthe compensation must be adjusted based on Δλ_(c) for the reduced radialregion.

By modifying the refractive index profile, we can adjust the speed ofthe guided modes to compensate for the effect of material dispersionbased on the distribution of wavelengths. As a result, an improved MMFhaving reduced total dispersion can be realized. The modifications tothe refractive index profile can be quantified using DMD waveform datawhich characterizes the modal propagation delays in the fiber.

We can quantify the material dispersion in MMF by measuring thewavelength-dependent time of flight of guided modes. One method, as usedherein, is to tune the wavelength of a titanium-sapphire laser used in aDMD measurement test bed. Clearly, other laser devices can be usedincluding tunable and fixed wavelength lasers. Since the maximumdifference in refractive index across the fiber core is small (<1%), ingeneral it is only necessary to characterize the wavelength-dependenttime of flight for fiber-coupled modes corresponding to one radialoffset.

With reference to FIG. 6, the time of flight is plotted for two radiallaunch offsets, 5 and 19 μm, propagating through a 550 m length of MMF.Each of the two curves in FIG. 6 quantifies the material dispersion ofthe fiber, n(λ). Longer wavelengths have shorter delay times andtherefore travel faster. The vertical shift in the curves is due to thedifference in modal propagation delay, or modal dispersion, between thetwo radial modes and is not related to material dispersion. The datashows the slopes of the two curves are almost the same. By convertingthe absolute time of flight data shown in FIG. 6 to relative delay inpicoseconds per meter (ps/m), we can relate the effects of material andmodal dispersion, where the delay due to modal dispersion is derivedfrom DMD waveform data.

With reference to FIG. 7, the delay due to material dispersion for the 5μm offset is plotted. The data points for the 19 μm are not shown inFIG. 7 since they are nearly identical.

The time of flight data in FIGS. 6 and 7 are for a test fiber thatexhibits significantly higher system performance than predicted bycurrent DMD and EMB metrics. With reference to FIG. 3, analysis of thefiber's DMD data shows that the high-order modes at 19 μm offsettraverse the fiber faster than the low-order modes at 5 μm, offset byabout 0.066 ps/m, which is the modal dispersion. When excited by theVCSEL transmitter, the modes for these two radial offsets (5 and 19 μmoffsets) will differ in center wavelength. Spectral analysis of thefiber modes when excited by the VCSEL used reveals that the modes at 19μm offset have a central wavelength of about 848.1 nm, whereas the modesat 5 μm offset have a central wavelength of 848.8 nm. From the time offlight (TOF) curve shown in FIG. 7, we see that this difference incentral wavelength corresponds to a difference in relative time delay of0.070 ps/m. However, since the optical spectrum of high-order modes at19 μm have shorter center wavelengths, on average the modes travelslower. Therefore, the negative material dispersion (−0.070 ps/m) willcompensate for the positive modal dispersion (+0.066 ps/m) reducing thetotal system dispersion to 0.004 ps/m. Hence, the fiber introduceslittle dispersion and performs better than predicted by conventional DMDand EMB measurement methods. This asymmetry in DMD waveform radial delayis not considered in the current, standard test methods.

For a given laser source or TOSA and radial variation in centralwavelength of the coupled fiber modes, we can calculate the relativedelay the fiber modes will undergo due to material dispersion. Therefractive index of the fiber can then be modified so that modes willtravel faster or slower, on average, to compensate for materialdispersion. For an emission pattern that emits shorter wavelengths intolarger angles (e.g., VCSELs), when coupled into the fiber, high-orderfiber modes will travel relatively slower than low-order modes. In thiscase, the refractive index must be reduced at larger radial offsets sothat high-order modes travel faster. The objective is to balance therelative delays of the guided fiber modes with the wavelength-dependentmaterial dispersion the modes will undergo so that the resultant totaldispersion is minimized. Once the relative delays required to compensatefor material dispersion are known, the necessary adjustments to therefractive index profile can be made. The required change in refractiveindex can be calculated by its relationship to the mode phase velocity,

$\begin{matrix}{{n(\lambda)} = \frac{c}{v(\lambda)}} & (4)\end{matrix}$

where c is the speed of light (299,792,458 m/s), and v is the mode phasevelocity (m/s).

Since each transceiver exhibits a unique spatial spectral distribution,the difference in radial spectrum central wavelengths (Δλ_(c)) must beestimated for a nominal transceiver that will minimize the effect ofmaterial or chromatic dispersion. An accurate understanding of thecoupling of VCSELs' wavelength-dependent spatial emission patterns intoMMF will lead to an improved design parameter. If different classes ofVCSEL transceivers (for example those used for Fibre Channel and thoseused for Ethernet) exhibit different radially dependent wavelengthemission patterns, the optimum fiber design parameters can be determinedfor each application. It may be practical to sort fiber for applicationspecific performance (“tailored” MMF).

As an example, the effect of material dispersion has been compensatedfor a randomly selected VCSEL having a Δλ_(c) of 0.53 nm. For thisVCSEL, the refractive index profile should be adjusted so that the DMDwaveforms peaks exhibit an overall relative shift in delay of about−0.04 ps/m from 0 to 24 μm, with shorter delay for larger radii. Thecompensation depends on both the Δλ_(c) and the radial region of thecore in which the modes are excited. One embodiment of the modifiedrefractive index profile is to design for a monotonic shift in delayacross the core of the fiber, as illustrated in FIG. 8A. The shift shownin FIG. 8A results in a 0.09 ps/m delay from 0 to 24 μm (a “left-tilted”shift). However, depending on the laser source wavelength emissionpattern and the fiber coupling characteristics, other radial shiftsmight be more appropriate. The calculated DMD waveforms (using the timedomain algorithm disclosed in U.S. Patent Application Ser. No.61/237,827) are shown in FIG. 8B. Close inspection of the calculatedwaveforms shows that the relative delays are nearly aligned, whichresults in low total dispersion.

To verify the new DMD specification, the new algorithm was applied tothe two sample fibers whose DMD waveforms are shown in FIGS. 9A and 9B.Based on the current test method, the measured DMDs and EMBs for thesetwo fibers are virtually identical (with an EMB of approximately 4543MHz*km), yet their measured Bit Error Rate (BER) system performancesdiffer by more than two orders in magnitude, where fiber (a) exhibitshigher system performance. We note that although the DMDs for these twofibers are the same, the peaks of their radial waveforms shift inopposite directions (delay) at larger radii (“left” vs. “right” shiftedfiber).

Using the algorithm, the calculated DMD waveforms for these two fibersfor the spectral characteristics of our BER test system VCSEL arepresented in FIGS. 10A and 10B. We see that the DMD waveforms in FIG.10A are more aligned than those in FIG. 9A. Whereas the DMD waveforms inFIG. 10B are shifted more to the right (at larger radii) than those inFIG. 9B. The minimum calculated bandwidth (minEMBc) for these two fibersare 3524 MHz*km and 2913 MHz*km predicting a 20% difference inbandwidth. The specified bandwidth of the fiber (EMB) is related tocalculated minimum EMB (minEMBc) by a factor of 1.13, i.e.,EMB=1.13×minEMBc. This difference in calculated bandwidth is alsoobserved in the new calculated DMD. Hence, the algorithm disclosed inU.S. Patent Application Ser. No. 61/237,827 correctly predicts theobserved difference in system performance between these two fibers.

Applying the algorithm to all fibers in a cable we can compare thestandard and predicted minEMBc and DMD (Inner Mask specification)metrics with BER system performance, as shown in FIGS. 11A, 11B, 11C,and 11D. The predicted metrics of FIGS. 11B and 11D, show a muchimproved correlation to measured system performance (R²=0.58 vs.R²=0.93) and DMD (R²=0.76 vs. R²=0.96).

FIG. 11A depicts a graph of measured minEMBc using a standard testmethod, in accordance with one embodiment of the present invention. FIG.11B depicts a graph of a newly calculated and convoluted minEMBc, inaccordance with one embodiment of the present invention. FIG. 11Cdepicts a graph of measured DMD, Inner Mask specification, using astandard test method, in accordance with one embodiment of the presentinvention. FIG. 11D depicts a graph of newly calculated and convolutedDMD, Inner Mask specification, in accordance with one embodiment of thepresent invention.

The new algorithm disclosed in U.S. Patent Application Ser. No.61/237,827 can be extended to the design specifications of MMF fiber bycharacterizing the shift in DMD required to compensate for the averageVCSEL and radial wavelength distribution in coupled fiber modes. Thecompensation is defined as the monotonic shift in the DMD waveform peaksacross the core of the fiber, 0 to 24 μm offsets. To determine the shiftneeded to compensate for the effect of Δλ_(c), the minEMBc arecalculated for a set of simulated fibers with different amounts oflinear DMD shift (as shown in FIG. 8A for a 0.09 ps/m shift from 0 to 24μm). With reference to FIG. 12, the calculated minEMBc (for threerepresentative transceivers selected to cover the sampled range in FIG.8) are plotted for varying degrees of DMD compensation. The curvelabeled “unconvoluted” depicts the decrease in minEMBc as we increasethe amount of compensation (shift in DMD waveforms). The standardalgorithm predicts a higher value of minEMBc when all the waveforms arealigned (zero DMD compensation for “unconvoluted” curve).

With reference to FIG. 13, by extracting the maximum new calculatedminEMBc for each of the curves in FIG. 12, the optimum DMD compensationfor a given transceiver (Δλ_(c)) can be determined.

Based on an analysis of our sample set of 10GBASE-SR complianttransceivers (18 devices) we should compensate for an average A) of 0.4nm. Using FIG. 13, the required compensation from 0-24 μm is −0.04 ps/mfor 10 Gbps Ethernet.

The DMD compensation for the sample fiber discussed above is close tothis predicted optimum compensation value of −0.04 ps/m (from 0-24 μm).The 3 dB bandwidth for this fiber was measured using 5 different VCSELtransceivers. The optical spectrum of each of the VCSELs was measuredacross the core of the fiber. The correlation between measured bandwidthand Δλ_(c) for a sample fiber using five different VCSEL transceivers isplotted in FIG. 14. The data verifies the maximum calculated bandwidthfor this fiber is obtained for a Δλ_(c) of 0.4 nm.

The new DMD specification disclosed herein should replace the current“ideal” DMD design metric. It is recognized that in practice, typicalDMD waveform plots exhibit multiple radial delay shifts and modesplitting as a result of variations in the fabrication process. Thisdoes not detract from the basic design requirement for a new targetrefractive index profile as proposed herein. Assuming we design for anominal transceiver, the fundamental requirement for improved MMF systemperformance is that the refractive index profile is biased such that theresultant DMD waveform plot shows a relative shift to lower propagationdelays at larger radial offsets (“left” shifted). One acceptable metricis to insure the difference in delay between the 19 μm and 5 μm radialoffsets is a negative number. Clearly, other radial offsets can be used;however we have found these values provide the best correlation tomeasured system performance.

For improved channel link performance it is proposed that the refractiveindex profile of laser optimized MMF be modified to compensate for theradially-dependent variation in center wavelength of the fiber modeswhen excited by VCSEL transceivers. For 10GBASE-SR compliant VCSELtransceivers, the refractive index profile should be modified so thatthe DMD waveform peaks exhibit a monotonic shift to shorter delays forincreasing radial offsets. The proposed shift in DMD waveform peaks is−0.04 ps/m over the range of 0 to 24 μm. This value compensates for theaverage VCSEL transmitter and wavelength distribution of coupled fibermodes.

It is understood that for VCSEL transceivers or other sources thatexhibit different radial dependent emission patterns, a differentcompensation will be required to correct for the radial variation inwavelengths. If the fiber-coupled modes exhibit a reversed radialdependent wavelength distribution (i.e., longer wavelengths coupled intohigh-order modes), the DMD compensation should be positive instead ofnegative. In general, any radial dependent wavelength distribution canbe compensated for reduced total dispersion.

Due to variations in the fabrication process, the dispersioncompensation for laser optimized MMF as specified herein (for 10 Gb/sEthernet or high-speed Fibre Channel) shall meet a DMD waveform profilerequirement of a left shift metric between 0 ps/m and 0.-0.14 ps/m forOM4 type MMF and 0 ps/m and −0.33 ps/m for OM3 type fiber, i.e., −0.14ps/m<(delay at 19 μm−delay at 5 μm)<0.0 ps/m for OM4 and −0.33ps/m<(delay at 19 μm−delay at 5 μm)<0.0 ps/m. In this manner, using thisDMD waveform profile requirement, MMF can be manufactured whichcompensates for material dispersions as well as modal dispersions.

Based on a representative sample population of 10GBASE-SR and 2G/4G/8GFiber Channel transceivers, it was empirically determined that the totaldispersion of a transmitter and fiber system can be minimized for a DMDdelay shift for OM4 type fiber between −0.01 ps/m and −0.04 ps/m.

In one aspect, the present invention provides for an optical transceivercomprising a transmitter optical sub-assembly (TOSA) that produces botha range of fiber-coupled optical power and a specified fiber-coupledspatial spectral distribution which compensates for material dispersionand modal dispersion effects.

Although any fiber coupled spatial spectral distribution withappreciable slope across the fiber core may result in materialdispersion that may be compensated with an appropriately designed fiber,one preferred embodiment of the present invention, is to couplehigh-order VCSEL modes into high-order fiber modes, as shown in FIG. 17.This can be achieved by requiring more precise components within theTOSA and more precise control of the TOSA alignment during themanufacturing process. Preferably, the alignment of components withinthe TOSA is to a tolerance which is within 1 mm or less, and thecomponents within the TOSA are manufactured to tolerances which arewithin 1 mm or less. Experimental data suggest this particularembodiment results in larger spectral distributions across the fibercore thereby offering a greater material dispersion effect that may beused for compensation.

Although it has been stated that material dispersion may be compensatedby modal dispersion, for moderate bandwidth laser optimized fibers witheffective modal bandwidths (EMBs) below 8 GHz·km and commerciallyavailable transceivers, the effects of modal dispersion are greater inmagnitude than material dispersion and so it may be customary to saymodal dispersion may be at least partially compensated by materialdispersion.

With reference to FIGS. 19, 20, and 21, a series of experiments arerepresented which quantify the fiber coupled spatial spectraldistribution for short wavelength, high bit rate, multimode fibertransceivers which are compliant with 10 Gb/s Ethernet and 8 Gb/s FibreChannel standards. In these experiments, an unmodified transceiver 142was powered up and modulated with either a 10 Gb/s or an 8 Gb/s PseudoRandom Binary Sequence (PRBS). The output of the transceiver 142 wascoupled by mating an MMF patch cord 148 with an appropriate connector(LC) to the transceiver 142. The end-face of the distal end of the MMFpatch cord 148 was aligned and then scanned with a single-mode fiber(SMF) patch cord 150 using a micro-positioning stage 144 and coupledinto an optical spectrum analyzer (OSA) 146. With this experimentalconfiguration, a fiber coupled spectrum was quantified across the coreof the MMF patch cord 148.

FIG. 20 illustrates measured fiber coupled spectrum at two differentfiber core radii: r=0 μm and r=24 μm for a particular transceiver. Note,that although the spectral components are largely similar, theirmagnitudes are not and therefore the calculated central wavelengths,λ_(c), are different. The λ_(c) at r=0 μm is 849.7 nm, while the λ_(c)at r=24 μm is 849.2 nm.

With reference to FIG. 21, a calculated change in central wavelength,Δλ_(c), (Δλ_(c)=λ_(c)−λ_(c minimum)) across a fiber core for twodifferent transceivers is provided. It is understood that other metricscan be used to characterize the fiber coupled wavelength variation.Fiber coupled modes for transceiver A exhibit a decreasing λ_(c) versusfiber core offset for transceiver B where the spectrum has a higheramplitude of shorter wavelengths. From this information, it may beinferred that components within the first TOSA for transceiver A areprecise and in good alignment. Conversely, the fiber coupled modes fortransceiver B exhibits an increasing λ_(c) versus fiber core offsetcorresponding to the situation in which lower-order VCSEL modes, arecoupled into higher-order fiber modes. Moreover, it can be inferred thatthe components within the second TOSA for transceiver B are impreciseand/or are not in good alignment. In summary, the magnitude anduniformity of the center wavelength variation depends on the VCSEL andoptical sub-assembly alignment. This fiber coupled spatial spectraldistribution, along with inherent dispersive properties of glass whichcause index of refraction to vary with wavelength, results in materialdispersion.

In traditional MMF, within the 850 nm wavelength region, the refractiveindex decreases with increasing wavelength and so shorter wavelengthradiation travels slightly slower, due to the increased refractiveindex, than longer wavelength radiation. Knowledge of a specific fibercoupled spatial spectral distribution may be used to determine both theeffects of material dispersion and also the specific amount (thedirection and the magnitude) of modal dispersion that may perfectlycompensate and nullify material dispersion. Therefore a fiber can beintentionally designed and fabricated to possess a particular amount ofmodal dispersion that will effectively balance the material dispersionresulting from a particular fiber coupled spectral distribution.Although the material dispersion effects resulting from a particulartransceiver/TOSA may be well compensated for with a specially designedfiber refractive index profile, it is commercially impractical tooptimize fiber and transceiver combinations individually; instead, thiscompensation must be performed en masse.

In an effort to better understand the range of transceiver fiber coupledspatial spectral distributions, the difference in fiber coupled centralwavelength between the inner fiber core region, 0 μm to 5 μm, and outerfiber core region, 19 μm to 24 μm, for thirty-eight short wavelength,high bit rate, multimode fiber transceivers was determined. Twenty-fourof the transceivers were compliant with 10 Gb/s Ethernet and fourteenwere compliant 8 Gb/s Fibre Channel. These transceivers weremanufactured by several suppliers including Finisar Corporation ofSunnyvale, Calif., Avago Technologies of San Jose, Calif., FiberxonInc., and JDS Uniphase of Milpitas, Calif. A histogram of this data isprovided in FIG. 22. The sign convention of Δλ_(c) was defined such thatif λ_(c) in the inner region, 0 μm to 10 μm, >λ_(c) in the outer region,11 μm to 24 μm Δλ_(c) is positive. Transceiver A in FIG. 21 hadΔλ_(c)=0.621 nm and Transceiver B in FIG. 21 had Δλ_(c)=−0.29 nm

Unfortunately, as is shown in FIG. 22, there is considerable spread inthe calculated delta central wavelength across the fiber core for thepopulation of transceivers tested and therefore, there is acorrespondingly large spread in dispersion that must be compensated forby the fiber's modal dispersion. Consequently, for well-compensatedsystems, numerous fibers would need to be designed and manufactured toaccommodate the diversity of this population. Moreover, since the fiberand transceiver are typically installed at different times it isimpractical to match a particular fiber with a particular transceiver.

An alternate, less advantageous, embodiment is to couple the VCSEL modessuch that the specified fiber coupled spatial spectral distribution hasminimal spatial dependence. This particular embodiment would realize aminimal total dispersion when the fiber had minimum modal dispersion andthe system would be limited by the effects of material dispersion.However, it should be noted that a well-compensated system would realizereduced total dispersion due to the fact that both modal and materialdispersive effects can effectively be compensated.

The advantages of this invention are that it will facilitate higherperformance optical links due to the fact that the total dispersion,including modal and material effects, can be minimized via precisedispersion compensation. Alternatively, due to the wide variation infiber coupled spatial spectral distribution, modal and materialdispersion can only be partially compensated by designing a fiber suchthat it optimizes the performance across the entire population.

With reference to FIG. 23, shown are calculated probabilitydistributions of the minimum optical link distances for: 1) linkscomprising TOSAs that do not specifically control fiber coupled spatialspectral distribution and therefore represented by FIG. 22 and a fiberdesigned to generally compensate for this wide distribution; and 2)links comprising TOSAs with specifically controlled fiber coupledspatial spectral distributions and a fiber designed to preciselycompensate for this material dispersion with modal dispersion.

As a result, one aspect of the present invention allows for a method forcompensating for both material dispersion and modal dispersion effectsin a multimode fiber transmitter optical sub-assembly. The methodcomprises measuring a fiber coupled spatial spectral distribution of themultimode fiber transmitter optical sub-assembly connected with areference multimode fiber optic cable and determining the amount ofmaterial dispersion and modal dispersion present in the referencemultimode fiber optic cable. Once the amounts of material dispersion andmodal dispersion present is determined, then a TOSA or an improved MMFcan be designed which compensates for at least a portion of the materialdispersion and modal dispersion present in the reference multimode fiberoptic cable resulting from the multimode fiber transmitter opticalsub-assembly. This allows for transmission of optical signals within theMMF with increased bandwidth.

While particular aspects of the present subject matter described hereinhave been shown and described, it will be apparent to those skilled inthe art that, based upon the teachings herein, changes and modificationsmay be made without departing from the subject matter described hereinand its broader aspects and, therefore, the appended claims are toencompass within their scope all such changes and modifications as arewithin the true spirit and scope of the subject matter described herein.Furthermore, it is to be understood that the invention is defined by theappended claims. Accordingly, the invention is not to be restrictedexcept in light of the appended claims and their equivalents.

1. A method for compensating for both chromatic dispersion and modaldispersion effects in a multimode fiber optic cable comprising:generating an optical signal into a reference multimode fiber opticcable; measuring a wavelength dependant time of flight for a pluralityof guided modes of the optical signal in the reference multimode fiberoptic cable; determining the amount of chromatic dispersion and modaldispersion present in the reference multimode fiber cable; and designingan improved multimode fiber optic cable which compensates for at least aportion of the chromatic dispersion and modal dispersion present in thereference multimode fiber optic cable.
 2. The method of claim 1, whereinthe differential mode delay (DMD) of the improved multimode fiber opticcable has a DMD designed to compensate for the chromatic dispersion andmodal dispersion in a fiber optic transmission system.
 3. The method ofclaim 1 wherein the measured shift in DMD waveform profile has a valuebetween −0.14 ps/m<(delay at 19 μm−delay at 5 μm)<0.0 ps/m for OM4 typemultimode fiber optic cable and −0.33 ps/m<(delay at 19 μm−delay at 5μm)<0.0 ps/m for OM3 type multimode fiber optic cable.